1. Field of the Invention
This invention relates to a high strength, low alloy, heat resistant steel which has excellent resistance to heat treatment cracks, i.e., it has a low susceptibility to cracks formed in welded joints during heat treatment after welding.
2. Description of the Related Art
Examples of well-known high temperature materials for use in heat resistant and pressure resistant piping for boilers, chemical equipment, and similar devices include low alloy, heat resistant steels containing a few % of Cr, high Cr ferritic heat resistant steels containing 9-12% of Cr, and austenitic stainless steels typified by 18% Cr-8% Ni steel. Of these materials, low alloy, heat resistant steel typified by 2.25% Cr-1% Mo steel (a so-called 2xc2xc% Cr-1% Mo steel) is inexpensive, so it is used in large quantities.
Low alloy, heat resistant steels containing a few % of Cr typically have a ferrite texture such as tempered bainite or tempered martensite. Compared to high Cr heat resistant steels and steels with an austenitic texture, their high temperature strength is low. For this reason, in recent years, there have been numerous proposals for improving the high temperature strength of low alloy, heat resistant steels by adding Mo, W, Nb, V, and other alloying element to a low alloy steel.
For example, Japanese Patent No. 2659813 discloses a steel which contains Cr: 0.7-3%, Mo: 0.3-1.5%, V: 0.05-0.35%, Nb: 0.01-0.12%, and N: 0.01-0.05%, and which is further adjusted to contain W: 0.5-2.4%, B: 0.0005-0.015%, Al: at most 0.1%, and Ti: 0.05-0.2%. The steel is heated to a temperature of at least 1100xc2x0 C. and is then cooled to room temperature, plastic working is carried out at room temperature or during working or during cooling in a temperature range in which recrystallization will not take place, and finally normalizing at a temperature lower than 1100xc2x0 C. and tempering at not higher than the AC1 temperature are carried out to manufacture a steel in which the degree of softening of weld heat affected zones is lessened and in which the impact properties of the base material are improved and which can be used at a temperature up to approximately 600xc2x0 C.
Japanese Published Unexamined Patent Application Kokai Hei 4-268040 discloses a low alloy, heat resistant steel having excellent creep strength and toughness and which contains Cr: 1.5-3.5%, W: 1-3%, V: 0.1-0.35%, Nb: 0.01-0.1%, B: 0.0001-0.02%, N: less than 0.005%, Al: less than 0.005%, and Ti: 0.001-0.1%, and if necessary further contains one or more of La, Ce, Y, Ca, Zr, and Ta each in an amount of 0.01-0.2% and Mg in an amount of 0.0005-0.05%, and further contains Mo: 0.01-0.4%, with the amounts of Ti and N satisfying the formula:
0.080xe2x89xa7Ti(%)xe2x88x92(48/14)N(%)xe2x89xa70.003.
Japanese Published Unexamined Patent Application Kokai Hei 5-345949 discloses a low Cr ferritic heat resistant steel having excellent toughness and creep strength which includes Cr: 1.5-3.5%, W: 1.0-3.0%, V: 0.10-0.35%, Nb: 0.01-0.10%, B: 0.0001-0.02%, N: less than 0.005%, Al: less than 0.005%, Ti: at least 0.001% and less than 0.050%, Cu: 0.10-2.50%, and if necessary contains Mo: 0.01-0.40% and one or more of La, Ce, Y, Ca, Zr, and Ta each in an amount of 0.01-0.20% and Mg in an amount of 0.0005-0.05%, and among impurities, P: at most 0.03%, and S: at most 0.015%.
This steel has a high creep strength, so the N content and the Al content are limited to less than 0.005%, and Ti is added to fix N, and B is added. In addition, in order to improve the toughness of welds, Ti, Cu, and W are added. Cu is added in order to improve oxidation resistance and corrosion resistance, while V, Nb, and W are added to improve strength.
Japanese Published Unexamined Patent Application Kokai Hei 8-325669 discloses an ultra-low Mn, low Cr ferritic heat resistant steel having excellent high temperature strength which contains Cr: 0.8-3.5%, W: 0.01-3.0%, V: 0.1-0.5%, Nb: 0.01-0.20%, Al: 0.001-0.05%, Mg: 0.0005-0.05%, B: 0.0020-0.02%, N: less than 0.005%, P: at most 0.03%, and S: at most 0.015%, and if necessary contains Mo: 0.01-1.5%, and one or more of La, Ce, Y, Ca, and Ta each in an amount of 0.01-0.2%, and a remainder of Fe and unavoidable impurities, wherein the B content satisfies the formula:
(14/11)B greater than Nxe2x88x92N(V/51)/{(C/12)+(N/14)}xe2x88x92N(Nb/93)/{(C/12)+(N/14)}.
In this steel, W is added to improve high temperature creep strength, the amount of Mn is limited to less than 0.1% in order to suppress a decrease in the effect of W after long periods of use, and B is added to prevent a decrease in strength and toughness and to increase high temperature creep strength. In order to guarantee the effectiveness of B, the amount of B is controlled by the above formula relating N, V, C, and Nb.
Japanese Published Unexamined Patent Application Kokai Hei 10-8194 discloses a ferritic steel having excellent weldability and high temperature strength which includes Cr: 0.3-1.5%, W: 0.1-3%, V: 0.01-0.5%, Nb: 0.01-0.2%, Al: 0.001-0.05%, B: 0.0001-0.02%, N: 0.001-0.03%, P: at most 0.025%, and S: at most 0.015%, if necessary one or more of Mo: 0.01-3%, Ca, Ti, Zr, Y, La, Ce, and Ta each in an amount of 0.01-0.2% and Mg in an amount of 0.0005-0.05%.
This steel is a low Cr ferritic steel which can be used in place of a high Cr ferritic steel and which has improved high temperature creep strength at a temperature of at least 450xc2x0 C. and which has performance comparable to or better than that of existing low alloy steels with respect to toughness, workability, and weldability.
When performing welding with low alloy, heat resistant steels like those described above, there is the problem that weld metal cold cracks develop. In order to prevent weld metal cold cracks, it is typical to perform preheating before welding and then to perform heat treatment after welding. However, as described on pages 10, 22-23, 100, and 150 of xe2x80x9cStandards for Heat Treatment after Welding and their Explanationxe2x80x9d (Japan High Pressure Technology Organization, edited by the Stress and Annealing Working Group, published by Nikkan Industrial Newspaper on Sep. 26, 1994), it is known that cracks develop in the weld heat affected zone of these low alloy, heat resistant steels during heat treatment after welding, i.e., they have a high susceptibility to heat treatment cracks. Heat treatment cracks are produced by a different mechanism from weld metal cold cracks, so they cannot be prevented by control of the preheating temperature.
Many reports have been published concerning heat treatment cracks in low alloy, heat resistant steels. For example, a heat treatment crack susceptibility index (PSR) is proposed in Journal of Welding Academy, Volume 41 (1972), No. 1, page 59. For a Cr content in the range of at most 1.5%, the heat treatment crack susceptibility index increases with increases in the amounts of Cr, Cu, Mo, V, Nb, and Ti. In particular, V, Nb, and Ti have a large effect on the index. In addition, Journal of Welding Academy, Volume 49, (1980), No. 3, page 203 discloses that the heat treatment crack susceptibility index increases as the amounts of the impurities P, Sb, Sn, and As increase in steel. Furthermore, Japanese Published Unexamined Patent Application Kokai Sho 59-80755 proposes a low alloy, heat resistant steel having excellent resistance to temper brittleness.
Each of the above-described publications concerning heat treatment cracks relates to a steel which does not contain W. As a result of studies by the present inventors, it became clear that in the case of a steel containing W, the strength at high temperatures is high, so the susceptibility to heat treatment cracking is markedly increased.
In the publications listed above, except for Japanese Published Unexamined Patent Applications Kokai Hei 4-268040 and Kokai Hei 5-345949, there is no description concerning welding cracks. In Japanese Published Unexamined Patent Applications Kokai Hei 4-268040 and Kokai Hei 5-345949, there is a description concerning prevention of weld metal cold cracks by control of the preheating temperature, but there is no mention concerning heat treatment cracking, which is a big problem with respect to steels containing W. Namely, at present, a high strength, heat resistant steel containing W and having adequate resistance to heat treatment cracking has not been obtained.
It is an object of the present invention to provide a Cr-Mo type high strength, low alloy, heat resistant steel which contains W and which has excellent resistance to heat treatment cracking.
The present inventors welded low alloy, heat resistant steels containing W and then investigated in detail the cracks which were generated in subsequent heat treatment (referred to below as post weld heat treatment). As a result, it was found that cracks are generated in heat affected zones where crystal grains are coarsened during post weld heat treatment at a temperature near the liquidus line. When the form of the fracture was observed with a scanning electron microscope, molten spots (cracks accompanying the formation of a liquid phase) were not found in the fracture, and as a result of analysis, a marked concentration of N on the fracture was found. Furthermore, as a result of observation with an electron microscope, it was found that minute carbides of V and Nb were generated within grains in the vicinity of cracks.
From these results, it was thought that weld heat treatment cracking is a phenomenon in which cracks open up due to the combined effects of factors such as the following:
(i) grain boundary segregation of N is accelerated by post weld heat treatment, and intercrystalline bonding strength is decreased,
(ii) due to precipitation hardening caused by carbides of V and Nb and solid solution hardening by W, the interior of grains is strengthened, and
(iii) deformation caused by thermal stresses is concentrated on the smooth surface of crystal grains which are coarsened due to welding heat cycles.
As a result of these observations, it was found that weld heat treatment cracks can be prevented by adjusting the form in which N is present by use of Ti and B. Namely, since Ti and B have a strong affinity for N, they form stable nitrides with N to decrease the amount of free N present at grain boundaries, the free N having the problem that it decreases intercrystalline bonding strength.
Ti generates TiN mainly at grain boundaries at the time of manufacture of steel, and due to the pinning effect, it suppresses coarsening of crystal grains caused by welding heat cycles. In order for this effect to be adequately exhibited, it is necessary for the amount of Ti to be at least 0.001%.
B has a strong tendency to segregate, so B which does not combine with N exists at grain boundaries as free B and occupies segregation sites. It thereby suppresses the segregation of N and other grain boundary weakening elements and increases intercrystalline bonding strength. As a result, it can contribute to the prevention of weld heat treatment cracks.
The effects of Ti and B in preventing cracks is of course strongly influenced by the amount of N, which is an intergranular embrittling element. When a large amount of N is present in a steel, large amounts of Ti and B are necessary to prevent cracks. FIG. 5, which was obtained using data from the below-described examples, illustrates the relationship between the amount of N present in a steel, relative to the amounts of Ti and B, and the occurrence of heat treatment cracks. In the figure, the abscissa is the N content [%N] of a steel according to the present invention or a comparative example, and the ordinate is the value of [%Ti]+5[%B]+0.004 for the steel. Steels in which there were no heat treatment cracks are indicated by an open circle (◯), and steels in which cracks occurred are indicated by a solid circle (xe2x97xaf). From this figure, it is ascertained that weld heat treatment cracks can be prevented, if the relationship between the N content [%N] and ([%Ti]+5[%B]+0.004) satisfies the following formula (1):
[%N]xe2x89xa6[%Ti]+5[%B]+0.004xe2x80x83xe2x80x83(1)
However, if the average crystal grain diameter in the heat affected zone exceeds 150 xcexcm, even if the above formula (1) is satisfied, the generation of weld heat treatment cracks cannot be prevented. In order to restrict the average crystal grain diameter in the weld heat affected zone to at most 150 xcexcm, the average crystal grain diameter of the base material must be at most 110 xcexcm.
Accordingly, the present invention provides a low alloy, heat resistant steel with excellent weldability which has an average crystal grain diameter of at most 110 xcexcm and consists essentially of, in mass %:
C: 0.03-0.15%, Si: at most 1%, Mn: at most 2%, P: at most 0.03%, S: at most 0.03%, Ni: at most 0.5%, Cu: at most 0.5%, Cr: 1.8-2.8%, V: 0.1-0.3%, Nb: 0.01-0.08%, Mo: 0.05-0.35%, W: 1.2-1.8%, Ti: 0.001-0.05%, B: 0-0.02%, Al: at most 0.1%; O: at most 0.1%, N: in an amount satisfying the formula
[%N]xe2x89xa6[%Ti]+5[%B]+0.004xe2x80x83xe2x80x83(1)
and a remainder of unavoidable impurities,
The average crystal grain diameter of the steel can be determined by counting the number of crystal grains present in an arbitrary length in a photograph taken using a microscope, and then dividing the length by the number of crystal grains.